Many large vehicles, of both off-road and over-the-road types, are equipped with air brakes. When compressed atmospheric air of varying relative humidity and initial atmospheric temperature is charged into such systems from a compressor, it contains moisture in the form of water vapor and condensed air-borne water droplets, as well as dust and other entrained air-borne foreign (non-air) matter. As the air delivered into the system from the compressor at above ambient atmospheric temperature cools to ambient atmospheric temperature, and as the entire system cools with decrease in atmospheric temperature, additional water vapor may condense in the system if conditions are right for such. Drain fittings are usually incorporated in such systems at locations where water, oil and other foreign matter tend to accumulate, so that upon opening of the particular drains, the water, oil and foreign matter are removed from the pressurized system at the drain points to preclude brake failure due to freezing of water in the air lines, or failure of brake parts due to the effects of water, oil or other foreign matter.
Devices popularly called Air Dryers are now incorporated between the compressor and the first air reservoir in many such systems to remove water, water vapor, oil and entrained particulate foreign matter. The air flow supplied by the compressor is by the dryer forced to flow downward and undergo deceleration and at the lower end of the dryer make a 180.degree. turn (to promote non-gaseous matter to fall out of the air stream into a trap) and then flow successively upward through an oil filter (to remove entrained oil and foreign matter), a desiccant cartridge containing about a pound of desiccant beads having about two million square feet of adsorbtive area (to adsorb moisture), through a desiccant seal plate, a purge chamber and a check valve into a line to a first reservoir. The system has a two-cycle operation. In the compression cycle, the compressor supplies compressed air through the dryer to the first reservoir until the desired system pressure is reached and the governor cuts out, pressurizing the unloader cavity of the compressor (placing the compressor in its non-compressing cycle) and pressurizing the purge valve in the bottom of the air dryer to open the valve to vent the air dryer sump trap to atmosphere. Contaminants in the line from the compressor to the sump trap of the air dryer are exhausted to the atmosphere through the open valve as the line pressure falls to atmospheric, while dried air in the purge chamber expands providing a reverse air flow across (down through) the desiccant to remove water and strip water vapor from the desiccant as the pressure in the air dryer falls to atmospheric. In use the desiccant becomes progressively saturated from bottom (air inflow from compressor end) toward the top until there is insufficient desiccant to adsorb moisture and water vapor from the air to dry it to the desired level. Before that level of operation is reached, the cartridge should be replaced. While for desiccant cartridge service life, preferred operating conditions are a compression cycle of 90 seconds or less, and a purge cycle of 30 seconds or more, the desiccant cartridge service life can vary widely with the actual operating conditions and atmospheric conditions under which it is used. In a dry dessert climate its life can be long, in a wet tropical climate it can be much shorter, and the service life in a vehicle operated in varying climates is not easily forecast. Similarly, leaks in a system or abnormal usage producing increased throughput of air, also affects the service life of the desiccant cartridge. However, the cartridge is located inside of a device having a steel case where it is not visible until the device, with the expenditure of some labor and time is disassembled. As a result, the timing of cartridge replacement to coincide with the end of cartridge service life has been a compromise of cost of labor, safety considerations and cost of waste of unused desiccant to arrive at a desiccant cartridge change schedule based on a factor such as operating hours, or miles driven, or a calendar period as assures change before exhaustion of the desiccant would occur under normal use, with perhaps a safety factor allowance included. The course of events, however, does not always follow such assumption based schemes and the practice is unreliable. If desiccant is changed unnecessarily, unnecessary cost results, but if it is not changed when it should be, fatal danger could be involved.